The present invention relates to the field of filling level indicators. It is based on a device and a method for filling level measurement.
In offshore oil production, separation tanks in which the phases occurring (sand, water, oil and gas) are separated by segregation and are carried away in separate line systems are used. For controlling the outlet valves, the filling levels, in particular the positions of the boundary layers between oil and water, are monitored.
High-pressure separation tanks designed for operation on the ocean bed, several 100 m below the surface of the ocean, have recently been developed. They allow the oil firstly to be separated and then to be pumped to the surface of the ocean with little expenditure of energy. Such separator tanks are subjected to very high pressures of 60 bar-180 bar and high temperatures of 50xc2x0 C.-120xc2x0 C. A filling-level measuring system must operate under such difficult conditions reliably and without maintenance for many years in order to ensure the operational safety of the separator tank.
Capacitive filling-level sensors which are immersed into the filled product and detect the filling level by changes in the dielectric constant and/or conductivity of the filled product are state of the art. A variety of electrode arrangements are known here. For example, in the German patent application with the application number DE 197 13 267.7 a rod-shaped closed measuring probe with a plurality of ring electrodes arranged along its probe axis is used.
A major problem when using capacitive filling-level sensors in separator tanks is that oil and oil constituents as well as process water and salt water represent a chemically highly aggressive and corrosive environment for the measuring probe and in particular for the electrodes. Materials which are potentially suitable for dielectric shielding, such as glass or ceramic, can, however, only be produced in the desired form with difficulty or with great expenditure.
It is the object of the invention to specify an improved dielectric protective coating which is stable over a long period of time and can be easily produced for a capacitive filling-level sensor.
According to the invention, the measuring probe of a capacitive filling-level sensor with at least one sensor electrode is protected by the measuring probe being sheathed at least partially by at least one outer layer of a fluorinated plastic and at least one inner layer of a mica-containing plastic. The combination of the fluorinated protective coating with the mica-filled protective coating has the effect of creating a chemically resistant and mechanically robust dielectric covering with a stable, small dielectric constant. The protective coating can be produced in small thicknesses (a few mm) and is suitable in particular for coating elongate measuring probes. The fluorinated plastic makes the outer layer hydrophobic and oleophobic. As a result, the adherence of water and/or oil residues on the measuring probe is reduced or eliminated and the reliability of the capacitance measurement is improved. The inner layer represents an additional diffusion barrier, especially to penetrating water. As a result, the constancy of the dielectric constant of the coating is ensured even when there is exposure to water over many years.
In a first exemplary embodiment, the outer layer consists of a thermoplastic material, such as for example perfluoroethylene-perfluoropropylene copolymer (FEP). On account of the thermoplasticity, the outer layer can be shrunk or extruded onto the measuring probe in a simple way.
In another exemplary embodiment, the inner layer consists of mica-coated glass fiber tape impregnated with epoxy resin and possibly silanized. This achieves particularly good water resistance. The tape form ensures that the inner coating is easy to produce by winding in one or more plies.
In a further exemplary embodiment, the measuring probe comprises a GRP pipe, which contains inside it glass spheres or polyhedrons and/or an inner rod of AlMgSi-filled epoxy resin and/or oil (for example mineral oil or silicone oil). The filling is suitable for high pressures, since it is largely incompressible and is thermally adapted to the GRP pipe.